Method of aluminum plating with diethylaluminum hydride



United States Patent 3,449,144 METHOD OF ALUMINUM PLATING WITH DIETHYLALUMINUM HYDRIDE 7 Billy J. Williams and Elbert L. Hatlelid, Ponca City, Okla., assignors to Continental Oil Company, Ponca City, Okla, a corporation of Delaware N0 Drawing. Filed Sept. 29, 1965, Ser. No. 491,387 Int. C1. C23!) 7/08; B44d 1/092, 1/34 U.S. Cl. 117--6 14 Claims This invention relates to a method of coating surfaces with aluminum. In one aspect, this invention relates to the use of titanium or a readily reducible titanium compound as a catalyst in liquid phase plating from a solution of diethylaluminum hydride.

Aluminium plating from liquid solutions or dispersions of aluminum alkyls offers some real economical means of plating substrates. Gas phase plating has been suggested; however, a number of practical problems are encountered. The system must be tightly sealed against atmospheric contamination, constant temperature conditions maintained and gas fiow rates controlled within narrow limits. Also, many problems exist in vaporizing decomposable materials and in almost all cases carrier gases must be provided to give direction to the flow of the decomposable gas. All additives to the gas stream, such as modifiers, must also be volatile at the operating temperature and the rate of addition carefully controlled. The article to be plated must be heated and prepared in the closed system which involves expensive process equipment.

To avoid some of the objections to gas phase plating, liquid phase plating has been proposed. For example, Berger (U.S. Patent 3,041,197) plates from a system wherein a heat decomposable aluminum compound in a liquid or solid solution or dispersion, wherein said liquid or solid solvent has a boiling point above the decomposition temperature of the aluminum compound, is utilized. The substrate to be plated is heated to a temperature above the decomposition temperature of the aluminum trialkyl, the decomposable aluminum compound, and is thereafter brought into contact with the solution or dispersion (hereinafter referred to merely as solution). Other agents may be present, for example, emulsifiers, wetting agents, reducers, oxidizers and the like. The substrate can be of any desired material so long as said substrate can be heated to a temperature above the decomposition temperature of the aluminum compound. It is also disclosed that the solvent affects the decomposition temperature, generally lowering the same. The deposited aluminum film thus plated can be anodized by conventional procedures.

In the copending application of Billy J. Williams and E. Flynt Kennedy, (Ser. No. 444,449, filed Mar. 31, 1965), it is disclosed and claimed that when the starting aluminum compound is a dialkylaluminum hydride, rather than the trialkylaluminums, superior plating is obtained in that the plating is much brighter than when the trialkylaluminums of Berger are employed.

In the copending application of Billy J. Williams (Ser. No. 444,392, filed Mar. 31, 1965), it is disclosed in liquid phase plating, superior plating of oxidizable substrates is greatly improved if the substrate, after cleaning is coated with a high boiling liquid hydrocarbon prior to heating. Such coatings apparently prevent any oxidation of the surface during the heating step. In this application, it is stated that the coating material is a heavy hydrocarbon having a boiling poin higher than the decomposition temperature of the aluminum alkyl. Such coating materials include the well known mineral oils and waxes. These mineral oils generally have a boiling range from 250 to 350 C. They are generally obtained from petroleum, generally by fractionation into various temperature range cuts and have no fixed chemical makeup depending upoin the nature of the original petroleum being fractionate In the copending application of Billy J. Williams and George J. Kostas (Ser. No. 444,431, filed Mar. 31, 1965), it is disclosed that when the aluminum alkyl is in solution having a boiling point below the decomposition temperature of the aluminum alkyl, further advantages are obtained over the higher boiling solvents of the Berger patent. Contrary to what would be expected, such solutions have less tendency to splatter and smoke than do the higher boiling solvents.

While the above methods have been effective in aluminum plating, for plating of effective thickness, several successive platings are frequently required.

It is an object of this invention to provide an improved method for plating substrates from solutions of diethylaluminum hydride and a novel plating solution.

According to the method of this invention, a small amount of titanium metal, titanium alloy or readily decomposable titanium compound is added to a plating solution of diethylaluminum hydride.

It has been found quite surprisingly that titanium promotes the plating action with diethylaluminum hydride whereas other transition metals show little or no activity. As will later be seen from the data, while nickel was effective at the higher bath temperatures, it was ineffective at the lower temperatures preferred with the low boiling solvents. It was also surprising to find that these titanium compositions promoted plating with diethylaluminum hydride without precipitation of aluminum in the bath, yet when dialkylal-uminum hydrides wherein the alkyl radicals contained more than two carbon atoms, aluminum was caused to precipitate in the bath, thus losing usable aluminum for plating.

As has been indicated above, any material which will not decompose or melt at the high temperatures required for preheating the substrate can be plated by the method of this invention. Of particular interest are metals which are subject to oxidation such as irons and steel to be plated to prevent rusting. Other metals such as copper, brass or even aluminum strips and the like can be plated by the method of this invention. Ceramics and glass can be coated, e.g., to make them electrically surface conductive or merely to improve the appearance. Metals thus coated can then be further plated with chromium or other metals such as by conventional electroplating methods. This is particularly advantageous in plating metals such as iron and steel which tend to be attacked by the electroplating solution. Also, organic or inorganic plastic or resins, which can be heated, without decomposing or melting, to above the decomposition temperature of the diethylaluminum hydride, can be plated by the method of this invention. Some phenolics, epoxy resins, and halogenated vinyl compounds fall into this class. Knowing the temperatures required, it is within the skill of the art to ascertain which materials would be suitable for such treatment.

The surface to be coated should be perfectly clean and free of any oxides. This cleaning can be mechanical such as by abrasion with steel wool or the like, sandblasting, sandpaper and the like. The cleaning can also be by chemical means such as ammonia solution of citric acid, dilute hydrochloric acid, dilute phosphoric acid and the like. With nonoxidizable surfaces, solvents or detergents can be employed. The chemical cleaning agent can be washed off with Water and immediately dried. The drying can be facilitated by use of drying agents such as acetone, methanol and the like.

With oxidizable surfaces, it is helpful to coat the substrate prior to heating with a heavy hydrocarbon having a boiling point higher than the decomposition temperature of the diethylaluminum hydride. Preferably, the

coating hydrocarbon should be compatible and miscible with the solvent for the aluminum compound. Such coatings include mineral oils and waxes. Such materials generally have a boiling range from 250 to 350 C. It would be expected, therefore, that the mineral oil or wax would be completely evaporated under the temperature of the furnace used for heating the substrate (usually 250-550 0, however, temperatures up to 1000 C. or higher can be employed). At these temperatures, one would expect to find decomposition of the coating material and fouling of the substrate. However, as disclosed in the aforementioned application of Williams, it has been found that no such decomposition occurs, and the substrate remains substantially free of oxide.

In applying the diethylaluminum hydride to the substrate, the substrate after coating (if desired) is preheated to a temperature in excess of the decomposition temperature of the aluminum compound, generally a temperature in the range 250-550 C. will be satisfactory. The substrate is then brought into contact with the solution of the aluminum compound by any convenient method. For example, the solution can be sprayed onto the substrate, the substrate can be dipped into the solution or the substrate can be passed through the bath. The high temperature of the substrate decomposes the aluminum hydride, causing aluminum to plate out onto the substrate.

Either high boiling solvents as disclosed by Berger can be utilized, or lower boiling solvents as disclosed by Williams and Kostas can be used. The Berger solvents include both normally liquid and solid state solvents, especially hydrocarbons. It should be understood that the normally solid state solvents will be at a temperature where they are liquid when the substrate is contacted therewith.

Solvents of the Berger type are higher alkanes, aromatics and parafiins which are stable and boil at a temperature greater than the decomposition temperature of the diethylaluminum hydride. Typical of such solvents are n-dodecane, 1,2,3,4-tetramethylbenzene, tetralin, naphthalene, l-methyl-naphthalene, diphenyl, anthracene, parafiin distillate, parafiin waxes, petrolatum and mineral oils.

We prefer the lower boiling solvents of Williams and Kostas, such as toluene, ethylene glycol monoethyl ether, ethylbenzene, xylene, kerosene and the like and particularly we prefer the hydrocarbons having the desired lower boiling point.

The use of various minor components in the solvent medium can cause modifications, where desired, of the physical and chemical properties of the aluminum film deposited. Such agents as wetting agents, oxidizing and reducing compounds etc. can be incorporated where desired. Representative examples include calcium phenyl stearate, polydimethylsiloxane, lead soaps, Na S O an- .4 hydrous hydrogen peroxide, KMnO LiAlH H peracetic acid and the like.

As an essential ingredient of the solution, we require a small amount of titanium or a readily reducible titanium compound. For example, as little as parts titanium by Weight per million parts aluminum has been found effective. In general, we use from 100 to 250 parts titanium per million weight parts aluminum since greater amounts of titanium do not increase the rate or thickness of the plating.

In addition to finely divided titanium, titanium alloys, such compounds as titanium tetraalkoxide, titanium tetraisopropoxide, titanium tetrabutoxide, tetrapentoxide and the like and titanium halides such as titanium dichloride, titanium trichloride, titanium tetrachloride, titanium dibromide, titanium tetrabromide, titanium trifluoride, titanium tetrafluoride, titanium diiodide, and titanium tetraiodide can be used.

The temperature of the plating bath can range over a wide range from atmospheric temperature up to the boiling point of the solvent. As a practical matter, the hot substrate will heat the bath and with a cold bath, higher substrate temperatures will be required. For that reason, we prefer an initial bath temperature of at least 50 C. and preferably above 100 C. In general, we employ a bath temperature in the range of 100 to 150 C.

In general, the aluminum alkyl in solvent can vary from 10-90 weight percent aluminum compound, but will preferably be employed in a 30 to 70 weight percent solution or dispersion.

Thus, typical plating baths would include 10% diethylaluminum hydride, 90% n-dodecane plus parts per million titanium powder; 20% diethylaluminum hydride, 80% tetralin plus 30 parts per million titanium tetrachloride; diethylaluminum hydride, 75% petroleum wash oil plus 45 parts per million titanium tetrachloride; diethylaluminum hydride, 70% kerosene plus parts per million titanium isopropoxide; diethylaluminum hydride, 50'% ethylbenzene plus 75 parts per million titanium tetrabromide; diethylaluminum hydride, 40% xylene plus parts per million titanium diiodide; diethylaluminum hydride, 10% ethylene glycol monomethyl ether plus parts per million titanium trichloride and the like.

To further illustrate our invention, several mild steel coupons (1 x 3 x 0.03 inch) were cleaned and coated with a thin film of mineral oil. A 51% diethylaluminum hydride and 49% petroleum wash oil was prepared and placed in beakers in a nitrogen-purged, dry box. To the diethylaluminum hydride were added various amounts of additive as indicated in the table of data. The additive is calculated in weight parts titanium or other metal per part of aluminum in the bath. The coupons were heated TABLE Grams cata- Furnace Weight alumi- Thickness Aluminum com- Catalyst m alyst per gram temperature, num plated, of plate, Run No pound terial aluminum 0. grams mile 0 350 0. 0008 0. 003 2X10 350 0. 0015 0. 006 2X10- 350 0. 0004 0. 001 0 350 0. 0003 0. 001 2X10 350 0. 0013 0. 005 2X10 350 0. 0004 0. 001 400 0. 0013 0. 001 2X10 400 0. 0041 0. 015 2X 10- 400 0. 0017 0. 006 0 400 0. 0013 0. 005 2X 10 400 0. 0022 0. 008 2X10 400 0. 0008 0. 003 0 450 0. 0031 0. 012 2X 10' 450 0. 0041 0. 015 2X10 450 0. 0035 0. 013 0 500 0. 0059 0. 021 2X10- 500 0. 0062 0. 023 2Xl0- 500 0. 0063 0. 023 0 500 0. 0054 0. 020 2x10 500 0. 0070 0. 026 0 500 0. 0012 0. 004 0 500 0. 0014 0. 004 2X10 500 0. 0010 0. 004 2X10- 500 0. 0016 0. 006

DEAH-Diethylaluminumihydride.

to various temperatures as indicated in the table and immediately dipped into the bath. The initial bath temperature was approximately room temperature or about 50 C. A strip was first dipped in a control bath, e.g. no additive, then a strip in a titanium tetrachloride-containing bath, and finally a strip in a bath containing nickel naphthenate. The process was repeated on down the series of runs so that by the end of the test, the bath had been heated to about 90 C. Growth product in the table refers to aluminum trialkyls obtained by reacting aluminum triethyl with ethylene to give aluminum trialkyls wherein the alkyls ranged from 8 to 22 carbon atoms. The data is shown in the above table.

In other tests the titanium (as titanium tetrachloride) was varied from 1 10 grams per gram aluminum to 4 l0- grams per gram of aluminum. There was no additional increase in aluminum plate thickness as the concentration of titanium was increased.

In still other runs, the plating solution as described above was blended with other metal salts instead of the titanium salt. These salts included cuprous chloride, cobalt chloride, vanadium trichloride, and zirconium tetrachloride. No catalytic effect was noted.

We claim:

1. A composition of matter comprising diethylaluminum hydride in a solvent therefore and a minor amount of titanium, titanium alloy or readily reducible compound of titanium.

2. The composition of claim 1 wherein the diethylaluminum hydride comprises to 90 weight percent of the composition and the equivalent titanium comprises at least 10 parts per million parts of aluminum.

3. The composition of claim ,2 wherein the solvent is a hydrocarbon.

4. The composition of claim 3 wherein the hydrocarbon has a boiling temperature below the decomposition temperature of the diethylaluminum hydride.

5. The composition of claim 4 wherein the diethylaluminum hydride comprises to weight percent of the bath.

6. The composition of claim 5 wherein the titanium is a titanium halide.

7. The composition of claim 6 wherein the titanium is in the form of titanium tetrachloride.

8. A method of aluminum plating a substrate which comprises heating the substrate to a temperature above the decomposition temperature of diethylaluminum hydride and thereafter contacting the heated substrate with a solution comprising 10 to weight percent diethylaluminum hydride, a solvent for said hydride comprising substantially the remainder of the composition and a minor amount of titanium, titanium alloy or a readily reducible titanium compound.

9. The method of claim 8 wherein the diethylaluminum hydride comprises 30 to 70 weight percent of said solution, and the titanium is present in at least 10 parts per equivalent titanium per million parts by weight of aluminum.

10. The method of claim 9 wherein the substrate is oxidizable and is coated with a heavy hydrocarbon having a boiling point higher than the decomposition temperature of diethylaluminum hydride prior to heating said substrate.

11. The method of claim 10 wherein the solvent of said solution has a boiling point below the decomposition temperature of the diethylaluminum hydride.

12. The method of claim 11 wherein the solvent is a hydrocarbon.

13. The method of claim v12 wherein the titanium is in the form of a halide.

14. The method of claim 13 wherein the titanium is in the form of titanium tetrachloride.

References Cited UNITED STATES PATENTS 2,698,811 1/1955 Legg 117-46 2,843,474 7/ 1958 Ziegler et al.

2,921,868 1/1960 Berger 117-107 3,041,197 6/1962 Berger 117l30X 3,294,654 12/1966 Norman et al. 117107.2 X

ALFRED L. LEAVITT, Primary Examiner.

I H. NEWSOME, Assistant Examiner.

U.S. Cl. X.R. 

1. A COMPOSITION OF MATTER COMPRISING DIETHYLALUMINUM HYDRIDE IN A SOLVENT THEREFOR AND A MINOR AMOUNT OF TITANIUM TITANIUM ALLOY OR READILY REDUCIBLE COMPOUND OF TITANIUM.
 8. A METHOD FO ALUMINUM PLATING A SUBSTRATE WHICH COMPRISES HEATING THE SUBSTRATE TO A TEMPERATURE ABOVE THE DECOMPOSITION TEMPERATURE OF DIETHYLALUMINUM HYDRIDE AND THEREAFTER CONTACTING THE HEATED SUBSTRATE WITH A SOLUTION COMPRISING 10 TO 90 WEIGHT PERCENT DIETHYLALUMINUM HYDRIDE, A SOLVENT FOR SAID HYDRIDE COMPRISING SUBSTANTIALLY THE REMAINDER OF THE COMPOSITIN AND A MINOR AMOUNT OF TITANIUM, TITANIUM ALLOY OR A READILY REDUCIBLE TITANIUM COMPOUND. 